Phase lock loss detector

ABSTRACT

A system for detecting phase lock loss between first and second two level signals S N  and S R , and comprising an Exclusive OR (XOR) gate having first and second input terminals and an output terminal, logic for supplying S N  and S R  to the first and second inputs of the XOR gate so that the center points of the respective upper and lower levels are aligned and with the time intervals between adjacent leading and trailing edges of S N  and S R  being nominally equal to Δt. Also provided is an AND gate having first and second input terminals and an output terminal, and delay logic for supplying the output signal of the XOR gate to the two input terminals of the AND gate with such output signal arriving at one of the input terminals of the AND gate a time interval Δt after the arrival of the output signal at the other input terminal of the AND gate and a detector for detecting an output signal from the AND gate to indicate loss of phase lock.

The U.S. Government has rights in this invention under sub-contract MIT/LL PO # Bx-435 pursuant to Government Contract u/F19628-80-C-0002 awarded by the Department of the Air Force.

This invention relates generally to digital frequency synthesizers and more particularly to a phase locked loop detector for determining when the phase of one signal, such as that generated by a voltage-controlled oscillator (VCO) is shifted with respect to the phase of a reference frequency signal by a predetermined amount.

There are many applications where it is desired to phase lock one signal with respect to a reference signal. Such systems are generally known as phase locked loop systems and employ a VCO which supplies a divided down output signal to one input of a phase detector. A reference signal is supplied to the other input of the phase detector and the output of the phase detector is supplied back to the frequency control input of the VCO to adjust its frequency so that the divided down frequency is phase locked with the phase of the reference signal. In many applications of this type circuit, it is desirable to know when phase lock between the two signals has been lost so that it can be reset by automatic means or, by a human operator who can intervene to remedy the failure.

There are many known phase lock loss detectors in the prior art. Many of these phase loss detector circuits are relatively complex and employ considerable logic. Some of them are adapted only to detecting phase lock loss where the two signals whose phase are being compared have the same duty cycle, usually 50%. Others are adapted to detect phase lock loss from duty cycles which are different.

The present invention provides a simple and inexpensive phase lock loss detector circuit which detects phase lock loss between two signals having either the same duty cycles or different duty cycles.

In accordance with a preferred form of the invention there is provided a system for detecting phase lock loss between first and second two level signals S_(N) and S_(R), and comprising an Exclusive OR (XOR) gate having first and second input terminals and an output terminal, for outputting a signal when the levels of the signals supplied to its first and input terminals are unequal, logic for supplying S_(N) and S_(R) to the first and second inputs of the XOR gate so that the center points of the respective upper and lower levels are aligned and with the time intervals between adjacent leading and trailing edges of S_(N) and S_(R) being nominally equal to δ. Also provided is an AND gate having first and second input terminals and an output terminal, and delay logic for supplying the output signal of the XOR gate to the two input terminals of the AND gate with such output signal arriving at one of the input terminals of the AND gate a time interval Δ after the arrival of the output signal at the other input terminal of the AND gate and a detector for detecting an output signal from the AND gate to indicate loss of phase lock.

In the drawings:

FIG. 1 is a block diagram of the invention;

FIG. 2 is a set of waveforms illustrating the operation of the circuit when the duty cycle of the two signals being phase locked are different;

FIG. 3 is a set of waveforms illustrating the operation of the invention when the duty cycle of the two signals being phase locked are substantially the same.

Referring now to FIG. 1, there is shown a typical system in which the invention is employed. The overall system is a phase locked loop including a VCO 10 which supplies an output signal to a divider 11. The frequency divided signal, arbitrarily defined (after phase lock), to be 5 MHz, is supplied to one input 16 of phase detector 12. A reference signal is generated in reference signal source 15 and also has a frequency of 5 MHz. The phase detector 12 responds to the two signals supplied to the two inputs 16 and 17 thereof to supply a control voltage E_(c) back to the frequency control input 13 of VCO 10 via lead 14.

The invention consists of the logic within the dashed block 35 which will be discussed first with the waveforms A-F of FIG. 2 and subsequently with the waveforms A-E of FIG. 3. For purposes of brevity, the waveforms of FIGS. 2 and 3 will be referred to herein as waveform 2A, 2B or waveform 3A, 3B, etc. rather than as waveform A of FIG. 2 or as waveform A of FIG. 3.

The waveforms of FIG. 2 represent a first mode of operation in which the duty cycles of signals (S_(N)) from the divider and signals (S_(R)) from reference source 15 are different. More specifically, the duty cycle of S_(R) has been selected to be 25%, i.e., the high level as shown in waveform 2A occurs during 25% of the time and the lower level 75% of the time. The duty cycle of S_(N) is 50%.

In a second mode of operation of the invention as represented by the waveforms of FIG. 3 the duty cycles of the signals S_(N) and S_(R) have been assumed to be equal, each being 50%. It is to be understood however, that duty cycles of both signals could be, for example, 25% or 40%.

Referring now to the first mode of operation as represented by the waveforms of FIG. 2, the signal S_(R), as shown in waveform 2A, is supplied to input 37 (see FIG. 1) of adjustable delay means 22 which has an adjustable delay time δ₁. The output of delay means 22 is supplied to a first input 20 of XOR gate 36. The signal S_(N), as shown in waveform 2C, is supplied directly to the other input 21 of XOR gate 36. Certain assumptions have been made with respect to signals S_(N) and S_(R). The first assumption, as mentioned above, is that the duty cycle of signal S_(R) is 25% and that of signal S_(N) is 50%. Furthermore, the leading edges of the two signals S_(R) and S_(N) are assumed to be in alignment under phase locked conditions, as shown at times t₁, t₅, t₉ and t₁₄ of the waveforms of FIG. 2. It is also assumed that the adjustable delay means 22 has been preset to have a delay δ₁ such that the output thereof, as shown in waveform 2B, is centered with signal S_(N) so that the time interval t₁ -t₂ (δ₁) is equal to the time interval t₃ -t₄ (δ) when the phase lock condition exists. The delay means 22 is thus seen to be equal to the time interval t₁ -t₂ and t₃ -t₄ so that the waveform of FIG. 2B is shifted a time interval δ with respect to the signal S_(R) of waveform 2A.

In the waveforms of FIG. 2, the phase locked condition is assumed to exist during the time period X under bracket 37 and that out-of-phase condition is assumed to exist during the time period Y under bracket 38.

It is a well-known characteristic of an XOR gate that when signals of unequal levels are supplied to the two inputs thereof, the XOR gate 36 will output a high level output (a 1 level) and when input signals of the same level are supplied the XOR gate 36 will output a low level output (a 0 level). Thus, during the phase locked condition during time period X, the output of XOR gate 36 will be as shown in waveform 2D and will consist of the four pulses 40, 41, 42 and 43 all of which have the same widths. Such pulses are the result of the waveforms 2B and 2C being of different levels during certain time intervals.

More specifically, during time periods t₅ -t₆ and t₇ -t₈ the two pulses 42 and 43 are generated during the unequal level portions of pulses 45 and 46 of waveforms 2B and 2C, and during time periods t₁ -t₂ and t₃ -t₄ the pulses 40 and 41 are generated during the unequal level portions of pulses 38 and 39.

The output of XOR gate 36, which is represented by waveform 2D, is supplied directly to the input 27 of AND gate 28 and is supplied to the other input 25 of AND gate 28 through adjustable delay means 26, which has an adjustable delay time of Δ₁, as represented by waveform 2E. It can be seen that the pulses of waveform 2E have been delayed by the time interval Δ₁ which is greater than the width of any of the pulses 40-43 of waveform 2D by a time interval α₁ and consequently, arrive at the input terminal 25 of AND gate 28 a time interval α₁ after the termination of the pulse of waveforms 2D. Specifically, the time interval Δ₁ is selected to be a time interval α₁ greater than the width of the pulses 40-43 in order to avoid a race condition at the two inputs of AND gate 28. Thus, the leading edges of output pulses 50, 51, 52 and 53 from adjustable delay means 26 each occur a time interval α₁ after the trailing edges of the four pulses 40, 41, 42 and 43 of waveform 2D. There is, therefore, no output from AND gate 28 during the time period X, as is shown in waveform 2F during the time period X.

Reference is now made to the performance of operation of the system during time period Y of FIG. 2 during which time period a disruption of the phase lock between signals S_(N) and S_(R) has occurred, as shown in waveforms 2B and 2C during time period Y. More specifically, the reference signal of waveform 2B has advanced with respect to the phase of signal S_(N) so that the time interval t₉ -t₁₀ is much less than the time interval t₁₁ -t₁₃. The pulses 56 and 57 of waveform 2D at the output of XOR gate 36 of FIG. 1 are a direct result of unequal levels of the signals of waveforms 2B and 2C during the time intervals t₉ -t₁₀ and t₁₁ -t₁₃.

The pulse 54 waveform 2E is the delayed input pulse to AND gate 29 from adjustable delay means 26 and can be seen to occur well after the termination of pulse 56 of waveform 2D which is supplied to the input 27 of AND gate 25. Therefore, no output from AND gate 28 occurs.

However, the pulse 55 of waveform 2E which is outputted from delay means 26 is seen to overlap the pulse 57 of waveform 2D which is supplied to the other input 27 of AND gate 28. Consequently, during the overlap period, time t₁₂ -t₁₃, AND gate 28 outputs a pulse 69 as shown in waveform 2F. This pulse is supplied to a latch or flip-flop circuit 30 in FIG. 1 which responds thereto to supply a signal to alarm 32. The alarm 32 functions to alert an operator or some suitable corrective circuit in the system.

The generation of pulses 59 and 60 of waveform 2E and of pulse 61 of waveform 2F similarly are caused by the shift in phase of the waveforms 2B with respect to waveform 2C.

Referring now to the waveforms of FIG. 3, there is shown the aforementioned second mode of operation wherein the duty cycles of signal S_(R) of waveform 3A and S_(N) of waveform 3B have the same duty cycle. During time period S it is assumed that phase lock exists and that the leading edges of pulses 72 and 73 of waveforms 3A and 3B are coincident as well as the leading edges of pulses 74 and 75. Since the duty cycles are the same, the trailing edges of the pulses 72 and 73 and of 74 and 75 are also time coincident as shown in FIG. 3.

The signals S_(R) and S_(N) of waveforms 3A and 3B are supplied respectively, to the inputs of adjustable delay line 22 (FIG. 1) and the input 21 of XOR gate 36. However, since the duty cycles are the same, there is no need to delay the signal of waveform 3B with respect to the signal of waveform 3A. Accordingly, the adjustable delay line 22 is preset to a 0 time delay so that, in effect, S_(R) is supplied directly to input 20 of XOR 36.

Since the two signals S_(N) and S_(R) supplied to the two inputs of XOR gate 36 are time coincident, there is no output from XOR gate since there is no time period in which the two input signals are of unequal levels. Since there is no output from XOR gate 36, the adjustable delay line 26 is not a part of the circuit and the inputs to the two inputs 25 and 7 of AND gate 28 are always both 0 level so that the output thereof is a 0 level and the latch 30 is never enabled.

However, during time period T, assume that the signal S_(N) of waveform 3B is delayed with respect to signal S_(R) of waveform 3A a time period t₂ -t₃ so that for such time period t₂ -t₃ the signal levels of S_(R) and S_(N) are unequal and XOR gate 36 outputs a pulse 78, as shown in waveform 3C. This pulse 78 is supplied directly to one input 27 of AND gate 28 and through delay means 26 to the other input 25 of AND gate 28. The delayed pulse is shown as a pulse 79 of waveform 3D and the leading edge can be seen to occur a time interval α₂ after the trailing edge of pulse 78. Therefore, there is no time overlap between the two pulses 78 and 79 of FIG. 3 and consequently, no output from AND gate 28 so that the latch 39 remains inactivated.

It is to be noted that the time delay Δ₂ of adjustable delay line 26 shown in the waveforms of FIG. 3 is not necessarily the same as the delay time Δ₁ indicated in FIG. 2 of the waveforms of FIG. 2. Nor are the delay times δ₂ and α₂ of the waveforms of FIG. 3 necessarily the same time delays as δ₁ and α₁ of the waveforms of FIG. 2. It can be seen that both α₁ and α₂ are the differences between the associated time delays of Δ and δ.

Similarly, pulses 83, 84, and 85 of waveform 3D occur completely after the pulses 86, 87 and 88 of waveform 3C so that there is no resulting output from AND gate 28.

Referring now to the time period W of FIG. 3, as indicated under the bracket 80 in FIG. 3, the signal S_(N) has been delayed even more with respect to the reference signal S_(R) of waveform 3A. Such phase lag is equal to the time interval t₄ -t₆ and is designated as delay β₂. The delay β₂ is equal to the width of pulse 90 of waveform 3C. Pulse 90 is supplied directly to one input 27 of AND gate 28 and through delay means 26 to the other input 25 of AND gate 28. The delayed pulse is shown as pulse 91 in waveform 3D and can be seen to time overlap pulse 90 by time interval t₅ -t₆. During this time interval t₅ -t₆ an output pulse is outputted from AND gate 28 and is shown as pulse 92 in waveform 3E. Such a pulse 92 functions to enable latch 30 and consequently, also enables alarm 32.

In a similar manner, pulse 94 of waveform 3E is generated due to the time overlapping of pulses 95 and 96 of waveforms 3D and 3C.

It is to be understood that the time delay values of adjustable time delays 26 and 22 depend upon the particular parameters of a given circuit. In the case of a signal having different duty cycles the adjustable delay 22 is adjusted to center signal S_(R) with signal S_(N) and the required delay is dependent not only upon the frequency (the periods) of two signals but also upon the relative duty cycle values.

With respect to the adjustable delay means 26, the value Δ, thereof is determined by the width of the pulses supplied from XOR gate 36 and the safety margin desired, i.e., the value the time delay between the trailing edge of the pulse supplied to input 27 of AND gate 28 and the leading edge of the pulse supplied to input 25 of AND gate 28. Such delay is designated as delay α₁ in FIG. 2 and α₂ in FIG. 3.

In the case of the mode of operation wherein the duty cycles of signals S_(R) and S_(N) are equal, the adjustable delay 22 is made equal to 0 for all conditions. The value of the time delay value of adjustable delay 26 again determines the margin of tolerance of phase difference desired and indicated by the time delay α₂ in FIG. 3. It is apparent from an examination of the waveforms FIG. 3 during time period W that if α₂ becomes less than the phase difference between signals S_(N) and S_(R) of waveforms 3A and 3B overlap will occur and the AND gate 28 of FIG. 1 will be energized to enable latch 30 and therefore, the alarm 32. 

What is claimed is:
 1. A system for detecting phase lock loss between first and second two level signals S_(N) and S_(R), of different ratios of high level portions to low level portions (duty cycles), and comprising:a single Exclusive OR (XOR) gate having first and second input terminals and an output terminal; means for supplying S_(N) and S_(R) to said first and second input terminals of said XOR gate so that the center points of the respective upper levels of S_(N) and S_(R) are aligned during phase lock; AND gate means having first and second input terminals and an output terminal; means for supplying output signals from said XOR gate to the first and second input terminals of said AND gate means with said output signals arriving at the first of said input terminals of said AND gate means a time interval Δ₁ after the arrival of said output signals at the second of said input terminals of said AND gate means, where Δ₁ is greater than the width of an output signal from said XOR gate when the center points of the upper levels of S_(N) and S_(R) are aligned; and means for detecting an output signal from said AND gate means when Δ₁ is less than the width of an output signal from said XOR gate.
 2. A system for detecting phase lock loss between first and second two level signals S_(N) and S_(R), where the duty cycles of the high level portions of S_(N) and S_(R) are different, and comprising:an Exclusive OR (XOR) gate having first and second input terminals and an output terminal; means for supplying S_(N) and S_(R) to said first and second inputs of said XOR gate with the center points of the upper levels being substantially aligned so that the time intervals between adjacent leading and trailing edges of S_(N) and S_(R) are both substantially equal to δ₁ during a phase locked condition between S_(N) and S_(R) ; AND gate means having first and second input terminals and an output terminal; means for supplying the output signal of said XOR gate to the first and second input terminals of said AND gate means with said output signal arriving at a first of said input terminals of said AND gate means a time interval Δ₁ after the arrival of said output signal at the second input terminal of said AND gate means; and means for detecting an output signal from said AND gate means.
 3. In a phase locked loop system comprising signal generating means including a voltage-controlled oscillator (VCO) for generating a two level signal S_(N) of frequency f_(N) and having a first duty cycle ratio of high level portion to total cycle period 1/f_(N), means for generating a two level reference signal S_(R) of frequency f_(R) and having a second duty cycle ratio of high level portion to total cycle period 1/f_(R) different from said first duty cycle ratio, phase detector means responsive to S_(N) and S_(R) for generating a correcting signal E_(c), said VCO responsive to E_(c) to change its output frequency so that f_(N) =f_(R) and with S_(N) and S_(R) being phase locked with a predetermined phase offset, a phase lock loss detecting circuit comprising:an Exclusive OR (XOR) gate having first and second input terminals and an output terminal for generating an output signal when the signal levels supplied to said input terminals are unequal; means including delay means for supplying S_(N) and S_(R) to said first and second inputs of said XOR gate so that the center points of the high level portions of S_(N) and S_(R) are nominally time aligned with any time difference 2δ₁ in the duty cycle ratios of S_(N) and S_(R) being substantially equally divided between the leading edges and the trailing edges of each half cycle of S_(N) and S_(R) ; AND gate means having first and second input terminals and an output terminal; means responsive to the output signal of said XOR gate to supply said output signal directly to the first input of said AND gate means and to said second input of said AND gate means delayed by a time interval greater than δ₁ ; and means for detecting an output signal from said AND gate means to indicate loss of phase lock.
 4. A method for detecting phase lock loss between two initially phase locked two level signals S_(N) and S_(R) having ratios of high level portions to low level portions which are different by a time interval 2δ₁ and comprising the steps of:aligning the centers of the high level portions of S_(N) and S_(R) ; XORing the aligned signals S_(N) and S_(R) ; delaying the XORed signal by an amount not less than δ₁ which is the amount of phase shift that can occur between S_(N) and S_(R) before phase lock is lost; ANDing the XORed signal and the delayed XORed signal; and detecting the output of said ANDing step. 